Recently, camera module-equipped electronic apparatuses such as cellular phones have been required to be smaller and thinner. Thus, a traditional package structure including a ceramic package, in which a solid-state imaging device is placed, and a surface-bonded glass sheet for sealing the package is failing to meet the size and thickness reduction requirements.
Thus, a flip-chip mounted package structure has been developed, which includes a microlens array and a glass sheet bonded directly onto the microlens array. For example, a relatively hard transparent material is provided as a protective layer to cover the on-chip microlenses. Such a protective layer can eliminate the need for a special package, and can reduce the number of processes for individual solid-state imaging device chips after dicing, so that the process can be simplified. In addition, the protective layer is relatively hard and has a planarized surface. Thus, even if dust is deposited on the protective layer, it can be easily wiped off with no scratches on the protective layer.
In recent years, as solid-state imaging devices have been made with a smaller size and a higher density of pixels, there has been the problem of sensitivity reduction, which is caused by a reduction in photoelectric conversion part area. To solve this problem, there has been provided a color solid-state imaging device having microlenses on photoelectric conversion parts.
In a conventional microlens structure, when the lens aperture diameter of the video camera is sufficiently small and when light is perpendicularly incident on the color solid-state imaging device, the incident light is converged on the photoelectric conversion parts with no problem. When the video camera lens is set close to full aperture, however, an oblique light component that cannot be converged on the photoelectric conversion parts increases, so that the color solid-state imaging device will have a problem in that its sensitivity is not effectively improved.
To solve this problem, there has been proposed a solid-state imaging device structure having microlenses and a planarized transparent resin material provided on the microlenses so that the uppermost surface of the solid-state imaging device is substantially flat. FIG. 7 schematically shows the structure of the solid-state imaging device having such a structure, which includes color filters and upper components. In the solid-state imaging device of this structure, a planarizing layer 102 is provided on color filters 101, and a microlens layer 103 is provided on the planarizing layer 102. In addition, a transparent resin layer 104 with a planarized surface is provided on the microlens layer 103.
Unfortunately, if there is only a small difference between the refractive index n1 of the microlens layer 103 and the refractive index n2 of the transparent resin layer 104, the light-converging function of the microlens layer 103 will be insufficient. Specifically, the collection efficiency of the light-collecting structure having microlenses and a planarizing resin film as shown in FIG. 7 is half or less of the collection efficiency of a traditional light-collecting structure having an air layer above the microlens layer 103.
There has been proposed a technique for solving the problem with the light collection properties, which provides a structure including microlenses and a transparent resin layer, wherein the microlenses have a refractive index higher than that of the transparent resin layer (see, for example, Patent Document 1). In this structure, the focusing performance of the microlenses can be kept at a satisfactory level even when the light-receiving surface is covered with resin or the like. More specifically, this technique includes forming microlenses by an etch back process using silicon nitride (SiN).
FIGS. 8A to 8C show a method of forming microlenses by an etch back process.
In the etch back process, a planarizing layer 102 is formed on color filters 101 as shown in FIG. 8A. An optically-transparent microlens layer 103 film is then formed on the planarizing layer 102 by plasma CVD (chemical vapor deposition), for example, using silicon nitride (SiN). A resist layer 105 is then formed on the microlens layer 103. As shown in FIG. 8B, the resist layer 105 is patterned into lens shapes by photolithography, and then heat-treated, so that a patterned resist is formed. As shown in FIG. 8C, the microlens layer 103 is then etched into lens shapes using the lens-patterned resist layer 105 as a mask.
In this structure, the microlenses made of a silicon nitride (SiN) film have a refractive index of about 2. The transparent resin with which the microlenses are covered is a resin with a refractive index of about 1.5, such as an acrylic resin, an epoxy resin, or a styrene resin. Thus, the microlenses offer reliable focusing performance. In the plasma CVD, the silicon nitride film needs to be properly formed at a temperature sufficiently lower than the heat-resistant temperature of the organic resin and the color filters.